1. Field of the Invention
The invention relates in general to a shift register, and more particularly to a shift register having a level controlling circuit capable of performing a level control operation via the charge storing ability of a capacitor.
2. Description of the Related Art
In the modern age, in which the technology changes with each passing day, liquid crystal displays have been widely applied to electronic display products, such as a television, a computer screen, a notebook computer, a mobile phone or a personal digital assistant. The liquid crystal display includes a data driver, a scan driver and a liquid crystal display panel, which has a pixel array. The scan driver sequentially turns on the corresponding pixel row in the pixel array and thus transfers the pixel data, outputted from the data driver, to the pixels so that the image can be displayed.
In the current technology, a shift register is often used to implement the scan driver capable of sequentially turning on the corresponding pixel row in the pixel array. FIG. 1 (Prior Art) is a circuit diagram showing a conventional shift register unit. Referring to FIG. 1, a shift register unit SR(n) generates a level control signal VC(n) by way of boot-strapping. The level of the control signal VC(n) is substantially greater than or equal to the level (VDD-Vth), wherein the level VDD is a high voltage level of the shift register unit SR(n), and Vth is the threshold voltage of a transistor TA, for example. The transistor TA outputs a high level clock signal CKZ as a scan signal SC(n) in response to the control signal VC(n), wherein the high level of the clock signal CKZ is equal to the voltage level VDD.
However, the conventional shift register unit SR(n) controls the operations of the transistors TA and TB according to the control signal VC(n). Thus, the circuit load to be driven by the control signal VC(n) is higher so that the level of the control signal VC(n) is lower. For example, the level of the control signal VC(n) is lower than the level (VDD-Vth). Consequently, the level of the scan signal SC(n) is substantially lower than the high voltage level VDD to cause the too low level of the scan signal SC(n) and thus deteriorate the frame display quality of the liquid crystal display.
In addition, the width-to-length ratio of the transistor TB is equal to about ten times of the width-to-length ratio of the transistor TC, and the transistor TC is biased as a diode. Consequently, an inverter INV formed by the transistors TC and TB may generate an output signal Vg, which is an inverse of the control signal VC(n), in response to the control signal VC(n). However, because the transistor TC has the smaller width-to-length ratio, the transistor TC needs to withstand the higher current, generated by the transistor TA, when the transistor TC is turned on. Consequently, the transistor TC may be damaged to cause the shift register unit to generate the malfunction and to shorten the lifetime of the liquid crystal display. Thus, it is an important direction in the industry to design a level controller and a shift register each having a long lifetime, and thus to lengthen the lifetime of the liquid crystal display and enhance the frame quality thereof.